The thermodynamic cycle for ligand binding and conformational change in integrins. The ΔG values are for reactions in the direction shown by the arrows. From the unliganded closed state in the bottom left, the induced-fit mechanism proceeds clockwise, and the preexisting conformational change mechanism with selection by ligand proceeds counterclockwise, to the liganded open state in the top right. The cycle for headpiece fragments can be modified for intact integrins on cell surfaces by adding additional conformational states such as those shown in Fig. 1.